Method for producing structures

ABSTRACT

A process for making microporous structures that can be used as a catalyst support. The microporous structures have high porosity and high thermal stability, combined with good mechanical strength and relatively high surface area. The process is useful for making titanium dioxide for catalyst structures for use for fuel cells, sensors, electrochemical cells and the like.

This application claims priority from U.S. Ser. No. 60/241,041, filedOct. 17, 2000, and U.S. Ser. No. 09/982,599 filed Oct. 17, 2001, theentire contents of each are incorporated herein by reference.

The present invention relates to a process for making microporousstructures for use as a catalyst support. In particular, the presentinvention relates to a process for making titanium dioxide suitable as acatalyst structure for fuel cells, sensors, electrochemical cells, andthe like. The present invention also relates to a catalyst structureformed from the process of the present invention.

BACKGROUND OF THE INVENTION

Different materials and processes are known for the manufacture ofcatalyst supports. High porosity and good physical strength are generalrequirements for such products. High temperature stability of thestructure is also required if the catalyst operates at elevatedtemperature.

U.S. Pat. No. 5,036,037 teaches a method to produce metal oxidecatalysts by pyrohydrolysis from solutions of chlorides, fluorides, ornitrates. The process makes particles with a mean size of 20-30 micronsand with a high specific surface area. The processing temperature is atleast 500° C. and generally higher than 700° C., to avoid the presenceof the anion (chloride or fluoride) in the oxide product. The productcan be used as such or can be further treated to give it the requiredphysical or chemical properties. Although this process is suitable forthe intended product, improvements to the process are desired.

Novel processes for the manufacture of titanium dioxide from aqueoussolutions have been disclosed in PCT Publications WO 01/00530, WO01/00531, and WO 01/12555, the relevant portions of which areincorporated herein by reference. In general, these applicationsdescribe the processing of an aqueous solution of a titanium salt byevaporation to produce an intermediate. The evaporation is conducted ata temperature higher than the boiling point of the solution, but lowerthan the temperature where significant crystal growth of an oxide phaseoccurs. In some embodiments, the evaporation may be conducted at atemperature higher than the boiling point of the solution but lower thanthe calcination temperature of the intermediate.

In the case of titanium solutions, the temperature generally ranges from120° to 350° C., and preferably from 200° to 250° C. The process ispreferably conducted by spraying, and can be accomplished in a spraydryer. The spray drying process produces thin-filmed spheres or parts ofspheres, with a diameter of about 1 to 100 μm, and a shell thickness ofabout 0.03 to 5 μm.

After calcination and milling of these spheres or parts of spheres, anddepending on the conditions of evaporation, the choice of additives, andthe conditions of calcination, ultra-fine nano-sized TiO₂ or,alternatively, pigment grade TiO₂ can be obtained.

There has been no suggestion, however, that such a process caneconomically and commercially produce catalyst structures made of metaloxides from salt solutions of the metals. The present invention istherefore directed to a process to economically produce catalyststructures or catalyst supports.

Accordingly, the present invention teaches a process to producecatalysts or catalyst structures with high porosity, high specificsurface area, high mechanical strength, and excellent thermal stability.In contrast to the method disclosed in U.S. Pat. No. 5,036,037, themethod of the present invention uses lower temperature equipment for thefirst step of the process and adjunction of chemical control additives.The method uses a combination of spraying, pressing, andcrystallization, which allows optimal control of the physicalcharacteristics of the product.

SUMMARY OF THE INVENTION

The present invention provides a process for making catalysts orcatalyst structures that comprises mixing an aqueous solution of a metalsalt and a chemical control agent to form an intermediate solution. Thesolution is preferably free of any precipitate.

The intermediate solution is then evaporated to form an intermediateproduct. The evaporation is conducted under conditions to achievesubstantially total evaporation. In particular, the evaporation isconducted at a temperature higher than the boiling point of the feedsolution but lower than the temperature where significant crystal growthoccurs. The evaporation may be conducted at a temperature higher thanthe boiling point of the solution but lower than the crystallizationtemperature of the intermediate. In a particularly preferred embodiment,the intermediate is an amorphous solid formed as a thin film andpreferably is spherical or part of a sphere.

The term “substantially total evaporation” or “substantially completeevaporation” refers to evaporation such that the solid intermediatecontains less than 15% free water, preferably less than 10% free water,and more preferably less than 1% free water. The term “free water” isunderstood and means water that is not chemically bound and can beremoved by heating at a temperature below 150° C. After substantiallytotal evaporation or substantially complete evaporation, theintermediate product will have no visible moisture present.

The intermediate product is then mixed with a binder to form a mixturethat is then dried. The drying can be performed in any suitable mannerbut is preferably air dried. The dried mixture is then pressed into adesired shape. Suitable desired shapes include, but are not limited todisks, full cylinders or hollow cylinders in the size range of a few mmto about 20 cm.

The shaped product may then be heated to a temperature of about 100° C.to remove any remaining moisture or volatile compounds and preventcracks during the heat treatment (crystallization) step.

The heat treated product is then crystallized by raising the temperatureto a temperature between about 500° C. to about 1300° C. for a period oftime from about 2 to about 24 h and then cooled to room temperature. Thecooled product is then washed by immersing it in water or dilute acid,heated to boiling and maintained at the boiling point for a period oftime from about 5 min to 2 h, to remove traces of any water-solublephase that may still be present after the crystallization step.

The product formed as a result of the process of the present inventionincludes a catalyst structure characterized by a porosity in the rangeof about 30% to about 70% and a thermal stability such that less than 5%dimensional change occurs upon holding the structure at a temperature ofabout 1100° C. in an oxidizing atmosphere for 8 hours. A preferredproduct is a titanium dioxide catalyst structure consisting ofneedle-shaped particles that are strongly bound together whileexhibiting a high porosity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general flow sheet of the process according to the presentinvention.

FIG. 2 is a scanning electron micrograph of one example of a catalyststructure obtained according to the process of the present invention.

FIG. 3 is a scanning electron micrograph of another example of acatalyst structure obtained according to the process of the presentinvention.

DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a generalized flow sheet of the process of thepresent invention is shown. With regard to FIG. 1, it has been foundthat some of the same processing steps and some of the equipmentdescribed in U.S. patent application Ser. Nos. 09/500,207, 09/503,365and 09/503,636, the entire contents of which are incorporated byreference, can advantageously be used to economically produce catalyststructures made of metal oxides, from salt solutions of these metals. Inthis regard, it is noted that these patent applications correspond toPCT publications WO 01/00531, WO 01/12555, and WO 01/00530,respectively. The contents of these publications are described above.

In accordance with one embodiment of the process of the presentinvention, a metal salt solution 2 is mixed with one or more chemicalcontrol agents 4 in mixer 10. The metal salt solutions are generallysulfates, chlorides, oxychlorides, nitrates, or mixtures thereof. Inthis regard, the metal forming the salt can be selected from the groupconsisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Al, Sn, Sb, Pb, Bi,and mixtures thereof. The metal salt solution may include the solublesalts of Y, Ca, Mg, and mixtures thereof. For example, when the metalforming the salt is Ti or Zr, the metal salt solution may desirablyinclude the soluble salts of Y, Ca, Mg, and mixtures thereof. Thesolution is preferably free of any precipitate or suspension.

The process is particularly suited to aqueous solutions of titanium andzirconium, more particularly solutions of titanium oxychloride andzirconium oxychloride. The chemical control agents can be, but are notlimited to chloride salts of sodium, potassium, lithium, aluminum, tinand zinc. Carbonate, fluoride, sulfate, silicate, borate and othersuitable salts of the aforementioned elements may also be used.Additionally, phosphoric acid and phosphate salts of the aforementionedelements may be used. Accordingly, the chemical control agent isselected from the group consisting of chloride salts, carbonate salts,fluoride salts, sulfate salts, silicate salts, borate salts andphosphate salts of sodium, potassium, lithium, aluminum, tin, and zincand phosphoric acid. In a preferred process, titanium oxychloridesolution is used as feed and sodium phosphate is used as a chemicalcontrol agent.

The mixed solutions are subjected to substantially total evaporation 20by contact with a hot surface or by spraying in a stream of hot gas toform an intermediate product. The substantially total evaporation isconducted above the boiling point of the solution and below thetemperature where there is significant crystal growth. The intermediateproduct is an amorphous hydrous oxide. The term “substantially totalevaporation” means that the resulting intermediate product contains lessthan 15% free water, preferably less than 10% free water, and morepreferably less than 1% free water. The term “free water” is understoodand means water that is not chemically bound and can be removed byheating at a temperature below 150° C. After substantially totalevaporation, the intermediate product will have no visible moisturepresent. Water and volatile products of the acid involved are vaporizedand may be recovered by any known process.

Preferably, evaporation by the spraying process is accomplished in aspray dryer. The use of a spray dryer allows the resulting physical andchemical characteristics of the solid product to be controlled within afairly narrow range through control of the operating parameters,including temperature, flow rate, concentration of the metal, and thetype and amount of chemical control agents. In general, the temperaturein the spraying chamber is in the range of about 100° C. to about 400°C. and the concentration in the feed solution is in the range of about10 to about 200 g/l of metal. Preferably, the temperature range is fromabout 150° C. to about 250° C. and the concentration range of the metalis from about 50 to about 150 g/l.

The intermediate product resulting from spraying in a spray dryer willbe composed of thin-filmed spheres or parts of spheres. The dimensionsof the spheres may vary over a wide range, from about 1 μm to 100 μm indiameter, and the shell thickness in the range from about 30 nm to about5000 nm, preferably from about 30 nm to about 1000 nm. This intermediateproduct may be called amorphous.

This intermediate product of the spraying process is further mixed witha binder 6 in the second mixing step 30 and optionally with surfacetreatment additives 8. The binder may include material effective toprovide surface treatment of the intermediate product such as surfacedeposition and surface modification. It may involve complexmicrostructure control that may be used to enhance desired properties(i.e. thermal catalytic properties, conductivity of the surface layer ormorphology of the microstructure). The binders have a lubricating actionduring the pressing process and are necessary to provide a well-finishedsurface. Binders must have good burnout properties during the heattreatment process.

The binder may be an organic or an inorganic binder. Inorganic bindersmay include but are not limited to metal salt solutions, salts,colloidal metals and complex mixtures thereof. Organic binders may benatural or synthetic. Natural organic binders may include but are notlimited to starch and its derivatives, cellulose ethers such ascaboxymethyl cellulose, hydroxyethyl cellulose, methyl hydroxypropylcellulose and soybean protein. Synthetic organic binders may include butare not limited to polymers or copolymers of styrene, butadiene, acrylester, vinyl acetate, and acrylonitrile. Polyvinyl alcohol, ammoniumpolyacrylate and sodium polyacrylate are examples of suitable organicbinders.

The optional surface treatment additives are generally similar to thechemical control agents. Accordingly, the optional surface treatmentadditives include, but are not limited to, chloride salts of sodium,potassium, lithium, aluminum, tin and zinc. Carbonate, fluoride,sulfate, silicate, borate and other suitable salts of the aforementionedelements may also be used. Additionally, phosphoric acid and phosphatesalts of the aforementioned elements may be used. Accordingly, thechemical control agent is selected from the group consisting of chloridesalts, carbonate salts, fluoride salts, sulfate salts, silicate salts,borate salts and phosphate salts of sodium, potassium, lithium,aluminum, tin, and zinc and phosphoric acid.

Thereafter, the product is dried in a drying step 40. Drying may beperformed in any suitable manner. The material to be dried may be placedon shelves in a drying oven, or it may be passed in continuous motionthrough a belt oven or a pusher oven. Drying may also be accomplished ina rotary kiln. Heating may be provided for example by an electricheating resistance, by a flow of hot air or by a flow of hot combustiongases.

The dried material is pressed into the desired shape in a pressing step50. The pressure applied may vary over a wide range, but is preferablyin the range from 2000 to 20,000 psi. A hydrostatic press may be used topress disks or cylinders. Special attachments may be used to makepressed material of different shapes.

The shaped product may then be heated to a temperature of about 100° C.to remove any remaining moisture or volatile compounds and preventcracks during the heat treatment (crystallization) step. The pressedproduct is then subjected to a heat treatment step 60 where the pressedproduct is further dried and slowly heated. In a preferred embodiment,the heat treatment is conducted over a period of at least about 2 h to atemperature where crystallization and crystal growth occurs.

This crystallization temperature varies with the nature of the materialserving as a catalyst structure, as well as with the nature of theaddition. The temperature and conditions of conversion are dependentupon the nature and amount of the additives. The temperature isgenerally in the range of about 500° C. to about 1300° C. for a periodof time from about 2 to about 24 h and then cooled to room temperature.The temperature is generally in the range of about 800° C. to about 120°C. for TiO₂ and in the range from about 1000° C. to 1300° C. for ZrO₂.The total time of a heat treatment cycle varies from about 2 hours toabout 24 hours.

The equipment used for heat treatment may be any kind of furnace withgood temperature control. Small productions may be made in a mufflefurnace. For larger productions, a continuous belt furnace with zones atdifferent temperatures is preferred.

After this heat treatment step, the product may be subjected to one ormore washing steps 70, which may include washing with dilute acid and/orwashing with water to remove soluble traces of the chemical controlagent that was introduced before the spray-drying step or to remove theoptional surface treatment additive introduced after evaporation butprior to the heat treatment step.

The washing steps may include a step where the heat treatment productand the washing solution are heated to boiling and maintained at theboiling point for a period of time from about 5 min to 2 h, to removetraces of any water-soluble phase that may not have reacted during theheat treatment step.

The macrostructure of the product may be further modified by additionalsurface treatments or depositions 80. A different metal oxide may bedeposited by chemical vapor deposition or from liquid phase solutions ofthe metal oxide precursor. This coated material may then be thermallytreated to mineralize the surface coating. The coating and curingprocess may be repeated several times until the desired coatingthickness is developed.

Without being bound by any theory, it is believed that the sprayingprocess under the conditions of the present process yields spherescomposed of a thin film of an amorphous solid that can be converted to acrystal structure with the required properties. Particularly, thecrushed and compacted spherical shells create, after crystallization, astructure that is both very porous and mechanically robust.

The process of the present invention produces catalyst or crystallinestructures with a porosity in the range from 30% to 70%. In addition,the structures formed from the process of the present invention arecharacterized by having a thermal stability such that less than 5%dimensional change occurs upon holding the structure at 1100° C. in anoxidizing atmosphere for 8 hours. The specific surface area may varyover a wide range, but is generally in the range of about 1 to about 10m²/g. The mechanical crushing strength of the catalyst structureproduced is at least 1 MPa (10 bar or approximately 145 psi). The sizeof the individual particles forming the porous structure is generally inthe micron range, from about 0.1 micron to 50 micron in length. Theparticles may have any shape, but are often needles with a width tolength ratio from to 1:1 to 1:20.

The size of these structures is not limited by the mentioned techniques.Further, the surface may be modified to include metal or metal oxides ormay be coated with other metal oxides and solid solutions. The processof the present invention also allows the creation of multilayeredcrystalline materials fused into a porous macrostructure that may beeasily swept by reactive gases or solutions.

The following examples illustrate, but do not limit, the presentinvention.

EXAMPLE I

A solution of titanium chloride containing 40 g/l Ti, 140 g/l Cl and0.2% Na₃PO₄.12H₂O was injected and subjected to substantially totalevaporation in a spray dryer. The spray dryer consists of a reactionchamber followed by bag filters and a hydrochloric acid absorptionsystem. The solution was injected at a rate of 3 liters/min through anatomizing disk. Gases from the combustion of natural gas, diluted withair to 580° C. were also injected around the disk. The outlettemperature of the spray dryer was 250° C. and the total gas flow about800 scfm. Reactor off-gases were sent to a bag filter to collect theTiO₂ product.

Ten grams of polyvinyl alcohol of molecular weight between 11,000 and31,000 was dissolved in 100 ml of water. The mixture was further dilutedinto 200 ml of ethanol, and combined with 100 g of the titanium dioxidespray dryer product. The resulting paste was dried in a drying oven at90° C., and then pressed into discs of 3.4 cm diameter under a pressureof 500 bar. The discs were further heat-treated according to thefollowing cycle:

-   -   4 h at 110° C.    -   gradual temperature increase of 5° C./min up to 1150° C. 4 h at        1150° C.    -   the furnace was turned off and the product allowed to cool

The discs were washed by immersing them in water and brought to boilingfor a period of 1 h.

The resulting product showed a monolithic porous structure consisting ofrutile crystals grown in place. This structure is clearly distinct fromwhat would be obtained by sintering of a powdered crystalline material.XRD analysis showed only pure rutile as the predominant solid phase anddimensional and density measurements indicated a void space fraction of0.59.

FIG. 2 is a scanning electron micrograph of the structure, enlarged 3000times. It shows a structure of elongated rutile crystals with across-section of about 1-2 μm and a length of about 5 to 10 μm.

EXAMPLE II

To a solution of titanium chloride containing 40 g/l Ti and 140 g/l Clwere added 0.67 g/l sodium silicate Na₂SiO₃.9H₂O. The resultant solutionwas injected and subjected to substantially total evaporation in a spraydryer in the same conditions as those given in Example I. The titaniumoxide product was recovered in a baghouse.

Further treatment was identical to the treatment corresponding toExample I, except that the crystallization temperature was 920° C.instead of 1150° C.

The resulting product showed a monolithic porous structure consisting ofrutile crystals grown in place. XRD analysis showed only pure rutile asthe solid phase. FIG. 3 is a scanning electron micrograph of thestructure, enlarged 3000 times. It shows elongated rutile crystals witha length of about 2 to 10 μm and a width to length ratio up to 1:10. Theaddition of sodium silicate generally produced more elongated crystalsthan the addition of sodium phosphate.

While there have been described what are presently believed to be thepreferred embodiments of the invention, those skilled in the art willrealize that changes and modifications may be made thereto withoutdeparting from the spirit of the invention. It is intended to claim allsuch changes and modifications that fall within the true scope of theinvention.

1. A process for the manufacture of structures comprising: a. mixing anaqueous solution of a metal salt and a chemical control agent to form anintermediate solution; b. evaporating the intermediate solution to forman intermediate product wherein the evaporating is conducted in acontrolled temperature process at a temperature higher than the boilingpoint of the solution but lower than the temperature where significantcrystal growth occurs; c. mixing the intermediate product with a binderto form a mixture; d. drying the mixture to form a dried mixture; e.pressing the dried mixture into a desired shape; f. crystallizing byraising the temperature to a range between about 500° C. to about 1300°C. for a period of time from about 2 to about 24 h and thereafter bycooling to room temperature; and, g. washing the product of step f. 2.The process of claim 1 wherein the metal forming the metal salt can beselected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W,Mn, Al, Sn, Sb, Pb, Bi, and mixtures thereof.
 3. The process of claim 1wherein the metal salt is selected from the group consisting of titaniumoxychloride or zirconium oxychloride.
 4. The process of claim 1 whereinthe metal salt is titanium oxychloride and wherein during thecrystallizing step the temperature is raised to a range between about800° C. to about 1200° C.
 5. The process of claim 1 wherein the metalsalt is zirconium oxychloride and wherein during the crystallizing stepthe temperature is raised to a range between about 1000° C. to about1300° C.
 6. The process of claim 1 wherein the chemical control agent isselected from the group consisting of chloride salts, carbonate salts,fluoride salts, sulfate salts, silicate salts, borate salts andphosphate salts of sodium, potassium, lithium, aluminum, tin, and zincand phosphoric acid.
 7. The process of claim 1 wherein the evaporationstep is conducted at a temperature between about 100° C. and about 400°C.
 8. The process of claim 1 wherein the evaporating is conducted byspraying.
 9. The process of claim 8 wherein the intermediate productcomprises a plurality of hollow spheres and parts of spheres.
 10. Theprocess of claim 9 wherein the diameter of the spheres is between about1 μm and about 100 μm.
 11. The process of claim 9 wherein the thicknessof the sphere is between about 30 nm and about 5000 nm.
 12. The processof claim 1 wherein the binder is selected from the group consisting ofinorganic binders, organic binders, and mixtures thereof.
 13. Theprocess of claim 1 wherein the washing is conducted by successivelyimmersing the product in water, heating it to boiling, and keeping it atthe boiling point for a period of time from about 5 min to 2 h.
 14. Theprocess of claim 1 wherein a surface treatment additive is mixed withthe intermediate product and binder.
 15. A titanium dioxide structurecharacterized by a porosity in the range of about 30% to about 70% and athermal stability such that less than 5% dimensional change occurs uponholding the structure at 1100° C. in an oxidizing atmosphere for 8 h.16. The structure of claim 15 wherein the structure comprisesneedle-shaped particles that are strongly bound together.
 17. Astructure made according to the process of claim 1 wherein the structurehas a porosity in the range of about 30% to about 70%.
 18. The structureof claim 17 wherein the structure has a thermal stability such that lessthan 5% dimensional change occurs upon holding the structure at 1100° C.in an oxidizing atmosphere for 8 h.
 19. The structure of claim 17comprising a plurality of individual particles forming the structurewherein the particles have a size in a longitudinal direction from about0.1 to about 50 micron.
 20. The structure of claim 19 wherein theparticles have a width to length ratio from about 1:1 to about 1:20.